The invention relates generally to optical amplifiers in optical communication systems. More specifically, the invention relates to optical amplifiers for large-capacity dense wavelength-division multiplexing (DWDM).
Current and future high-speed, high capacity dense wavelength-division multiplexing (DWDM) communication systems have to handle two particular types of user services: multimedia services to multiple users, and select-cast data transport from user-to-user or from region-to-region. A dynamic reconfigurable multi-wavelength channel add/drop function at the user nodes can efficiently process the information of these two types of services, with minimum electronics at the access node, at lower system cost [see for example A. R. Moral et al. xe2x80x9cOptical Data Networking: Protocols, technologies, and architectures for next generation optical transport networks and optical internetworksxe2x80x9d, J. LightWave Technol. vol. 18, 2000 pp. 1855-1870]. Fiber optical amplifiers will be used in these WDM networks to compensate for insertion loss of optical switches and transmission loss in optical fibers. When the network is reconfigured and wavelength channels are added or dropped, cross-gain saturation in fiber amplifiers will induce power transients in the surviving channels, which can cause service impairment not known in electronically switched networks. As fiber amplifiers saturate on a total power basis, addition or removal of channels in a multi-wavelength network will tend to perturb other channels that share all or part of the route. The power of the surviving channels should be maintained constant in order to prevent unacceptable error bursts if the surviving channel power becomes too low to preserve adequate eye opening or exceeds thresholds for optical nonlinearities.
For DWDM applications, all channels going through the same amplifier gain medium should have as low a noise figure (NF) and as high a gain as possible. In addition to gain and NF requirements, WDM amplifiers must also conform to tight specifications with respect to multichannel gain flatness, gain-tilt, and channel add/drop response. Gain variation is the main concern in designing amplifiers because the gain profile of an optical amplifier changes with its gain. Existing commercial gain-flattened DWDM amplifiers in the current market typically use passively gain-flattening filters. Passively gain-flattened DWDM amplifiers are usually designed for a specific gain requirement, i.e., a well-defined output level for a well-defined input level. They often operate under automatic gain control in the system, meaning that when the input is changed, the output is also changed proportionally, with the gain remaining fixed. This feature fits well in systems where the power level of all channels is fixed but not in cases where channels are added or dropped from an amplified system.
In many situations, the channel-power that is input into a DWDM amplifier, is not constant. If channels have to be switched, re-routed, or transported from one point to multiple points, then the channel power arriving at the entrance of a DWDM amplifier is not constant. If there is to be no degradation in system performance, then all channels must be at approximately the same power level at the DWDM amplifier output, independent of the input power. Thus, a DWDM amplifier must be able to provide a variable gain, without affecting the amplification uniformity across all channels. Alternatively, there may be situations where the input levels do not change, but instead the channels may be required to be routed along a different path with a larger loss. In such cases, the flexibility of increasing the amplifier gain may be required, again without compromising the gain uniformity. The problem is that for a passively gain-flattened DWDM amplifier, if gain changes over the certain small dynamic range, gain shape will change and the corresponding NF may increase. A passively gain-flattened amplifier is inadequate for the varying and demanding DWDM environment.
To solve the above problems, the DWDM amplifier must be actively gain controlled. Many dynamic gain-flattened DWDM amplifiers have been investigated recently [S. K. Yun, et al., Dynamic erbium-doped fiber amplifier based on active gain flattening with fiber acousto-optic tunable filter, IEEE Photon. Technol. Lett., vol.11, 1999, pp.1229-1231]. [B. J. Offrein, et al., Adaptive gain equalizer in high-index-contrast SiON technology, IEEE Photon. Technol. Lett., Vol.12, 2000, pp. 504-506]. [J. C. Chiao, et al., Liquid-crystal optical harmonic equalizers, The Proceeding of the 27th European Conference on Optical communication, October, 2001]. [K. Wundke, et al., A fiber-based, slope adjustable filter for EDFA gain tilt control, The Proceeding of the 27th European Conference on Optical communication, October, 2001]. [T. Kitabayashi, et al., Novel gain-tilt free L-band EDFA using Thulium-doped fiber, The Proceeding of the 27th European Conference on Optical communication, October, 2001]. All the above methods have limitations both in the dynamic gain range and the response time, which makes them unsuitable the future DWDM networking systems.
Dynamic gain-flattened fiber amplifiers with ultra-wide dynamic gain range and very fast response time across an operational wavelength range with a very flat wavelength response regardless of channel count or channel power level are provided. The maximum variable range of the gain level is preferably 30 dB or larger with very fast response time across the whole C- or L-band wavelength range.
One broad aspect of the invention provides a switchable dynamic gain-flattened optical amplifier with a wide dynamic gain range. An optical signal is first amplified through common amplification such that the gain is approximately common to all channels of the optical signal. Further amplification is then achieved through distinct amplification wherein the optical signal is routed through one of N parallel amplification paths each having its own fixed gain. Each distinct amplification of N parallel paths has a passive gain flattening filter (GFF) to flatten the output power profile across the whole operational wavelength range within a certain flatness requirement (for example,  less than xc2x10.5 dB). Then the amplified signals are passed through a common variable optical attenuator (VOA) preferably having an attenuating range from 0 dB to L-dB.
Preferably, the value of the gain of the common amplification plus values of the fixed gain on the paths have been designed to satisfy the following relationship: G1=G2xe2x88x92L=G3xe2x88x922L= . . . =GNxe2x88x92(Nxe2x88x921)L, where Gi is the common gain plus the fixed gain of the i-th path (i=1, 2, 3, . . . N) and L is the maximum attenuating range of the common VOA. The total adjustable gain range of the dynamic gain-flattened optical amplifier in this case will be NL.
In order to self-adjust quickly and respond to changes in input conditions and/or operating conditions of the optical amplifier and output requirements while maintaining gain flatness and a low noise figure (NF) over a broad optical bandwidth and a wide range of gain levels, the switchable dynamic gain-flattened optical amplifier preferably makes use of two optical switches, one at the input to the N parallel amplification paths and one at the output of the N parallel amplification paths, to allow switching in and out one of the gain-flattening filters and gain mediums in parallel.
Preferably, a control function is provided to control the switchable gain amplifier. This involves controlling which of the paths an input signal should be routed through, and involves controlling the gain of the variable optical attenuator. A required overall gain may be input from a networking management system, and the control function makes adjustments to the switchable gain amplifier to best achieve the required overall gain. It may be necessary to control pump light source powers as well.
In a preferred embodiment, each time the control function detects an input level, it compares the input level with a preprogrammed look-up table and switches to a corresponding m-th amplification path. The control function may for example, make use of embedded software to control the common VOA to control the adjustable gain range from Gm to Gm-L within a very short time period preferably less than 1 ms.
In accordance with a first broad aspect of the invention, provided is a method of amplifying an optical signal. The method comprises first amplifying the optical signal. The optical signal is then further amplified through a selected one of a plurality of parallel amplification paths each having its respective fixed gain.
The method may further comprise performing gain equalization of channels of the optical signal in a respective one of the parallel amplification paths.
A variable gain is applied, for example with a common VOA, to dynamically control the gain within a certain range for the selected one of the parallel amplification paths. This may be done in response to changes in at least one of input conditions, output requirements, and operating conditions of an optical amplifier responsible for a respective one the first and further amplifications. Such dynamic control might further comprise switching the optical signal through a different one of the parallel amplification paths and/or changing the attenuating values of the VOA.
Another broad aspect of the invention provides a switchable optical amplifier. The optical amplifier comprises at least one common gain section and at least one switchable distinct gain section connected to receive an output of the common gain section. The switchable distinct gain section has a plurality of parallel amplification paths each having distinct gain characteristics.
In some embodiments, the common gain section might comprise an erbium-doped fiber amplifier (EDFA). In such embodiments at least one of the common gain section and the switchable distinct gain section may comprise a pump light source.
The parallel amplification paths may comprise a plurality of sections of the erbium-doped fiber (EDF) and in such a case the optical amplifier may comprise a pump light source, which is common to the plurality of sections of EDF. In such a case, each section of the erbium-doped fiber may have a different length.
A common VOA (variable optical attenuator) is preferably provided at the output of the switchable gain section for allowing a range of variability in the overall gain when a particular path in the distinct gain section is selected.
Each one of the pluralities of parallel amplification paths may comprise a respective gain flattening filter. In addition, each one of the plurality of parallel amplification paths in combination with both the common VOA and the common gain section may be adapted to provide a respective dynamic gain range with a desired gain flatness and a low noise figure over a broad optical bandwidth for a certain input/output power range.
The switchable distinct gain section may have N parallel amplification paths. In such a case the switchable distinct gain section may further comprise a 1xc3x97N input optical switch. Such an optical switch is adapted to connect the common gain section to any particular one of the N parallel amplification paths. In addition, in such a case, the optical amplifier also comprises an Nxc3x971 output optical switch that is adapted to connect the N parallel amplification paths to the common VOA, which connects to a common output.
The optical amplifier may be adapted for use as a C-band DWDM amplifier. In another embodiment, the optical amplifier further comprises an additional section of erbium-doped fiber between the common gain section and the parallel amplification paths. The additional section of the erbium-doped fiber is adapted to receive pump light from the pump light source, which causes inversion in the additional section of the erbium-doped fiber. The inversion results in further amplification of the optical signal and the generation of ASE a forward component of which might acts as a pump source together with the original pump source in the parallel amplification paths. Such an optical amplifier might be adapted for use as an L-band DWDM amplifier.
Preferably, the optical amplifier further comprises a control function that might be any suitable combination of hardware and/or software. Broadly speaking, the control function is responsible for selecting one of the parallel amplification paths. In some embodiments, it may be further adapted to adjust gain characteristics of the common optical amplifier section(s) and the switchable distinct gain section to achieve constant locked gain, which depends on the distinct passive GFF.
In another embodiment the control function is adapted to adjust the gain characteristics through a control pattern with a control speed that avoids optical transience during channel add/drop. The control function might also be further adapted to adjust the common VOA or to tune gain characteristics of at least one of the common gain section and the switchable distinct gain section to achieve gain-tilt-free operation and/or low noise figure. In some embodiments, the control function may be further adapted to dynamically adjust pump light source power to achieve at least one of said constant locked gain, gain-tilt-free operation and the low noise figure. In yet other embodiments, the control function may be further adapted to dynamically control a common VOA to achieve the desired gain from networking management systems. In such embodiments, this might be done in response to at least one of the input and/or output variations, new output requirements and changing operating conditions within the optical amplifier.
In order to facilitate this control, preferably the optical amplifier has an input asymmetric tap coupler that is adapted to route a portion of an input optical signal as a subsidiary input optical signal to the control function for monitoring input conditions. The optical amplifier might also comprise an input photodiode detector adapted to convert the subsidiary input optical signal into an electrical signal. Preferably the optical amplifier also has an output asymmetric tap coupler adapted to route a portion of an output optical signal as a subsidiary output conditions. The optical amplifier might convert the output optical signal into an electrical signal for use by the control function.
The parameters may be adjusted to provide at least one of a control loop for a constant locked gain, gain-tilt free operation and a low noise figure. The instructions may comprise switching an optical signal through one of a plurality of parallel amplification paths. The instructions might also comprise adjusting the VOA and/or the pump laser sources.
Advantageously, the invented optical amplifier is highly effective, in dense wavelength-division-multiplexed (DWDM) systems, in compensating for changes in operating conditions due to link loss change, pump deterioration, channel add/drop, and network reconfigurations.